Cashew leaf and nut blight – A new disease of cashew in Tanzania caused by Cryptosporiopsis spp.

In August 2002 a new and damaging leaf and nut blight disease was observed on young tissues of cashew (Anacardium occidentale) in southern Tanzania. Angular lesions, dark tan with a dark reddish brown margin, were formed on leaves, often vein limited and containing conidiomata. Lesions subsequently enlarge and coalesce causing blighting and defoliation. Older lesions become papery, silver/grey in colour and develop shot-holes. During fruit setting, infection of young nuts causes rapid blackening and abscission, resulting in significant yield losses. Infection of older nuts results in a characteristic dark, slightly sunken, 'tar spot' like lesion that frequently extends onto the apples. Under favourable conditions white spore masses of the fungus were observed within the nut lesions.

Leaf spots on cashew caused by Cryptosporiopsis sp.

Close up of leaf lesions caused by Cryptosporiopsis sp.

Leaf blighting under severe infection.

At the ARI, Tanzania field observations in 2002-2003 indicated that the disease is most active during wet weather, especially during off-season rains, where severe infections affect the young flushing material. Surveys confirm that the fungus is present throughout the Mtwara region, at research plots as well as on farmers' fields. Diseased plant materials were sent to the Global Plant Clinic in October 2003, where a new and undescribed Cryptosporiopsis species (Sutton, 1980) was consistently isolated from the nut and leaf lesions. One isolate has been deposited to the CABI Bioscience Culture Collection (IMI 391611).

Wounded and non-wounded leaves of cashew were inoculated with an aqueous conidial suspension (1×10⁶ conidia per ml) or with water (control) and maintained at 24-28°C and 95% RH for 24 hours. All inoculated plants showed leaf lesions similar to those observed in the field within 7-10 days after inoculation. None of the control plants developed any symptoms. From leaf lesions a Cryptosporiopsis species was re-isolated that was identical morphologically to the original isolate inoculated.

Cryptosporiopsis species are predominantly stem pathogens of temperate woody hosts, including maple, hazel and fruit trees (Old et al., 2002) and often have Pezicula teleomorphs (Sutton, 1980). Comparatively few reports of Cryptosporiopsis species have been received from tropical regions and this is first record of this genus attacking cashew. The severe blighting of nuts that can occur, particularly during off-season rains, poses a severe problem to cashew growers. The crop is an important part of Tanzania's export and contributes 10% of the total value of their foreign exchange earnings.